yeah, weather prediction in general is not doing so well. they will usually tell you it's "partly cloudy with a chance of rain" which when you think about it, when is it not partly cloudy with a CHANCE of rain. Anyway. depending on where you are sailing there can be better ways to know the winds. For example, I sail on the ocean and around the area I go the wind and sea are both pretty calm in the second half of comming from high tide to low tide. Then the first half of low tide to high they become steady, but not too fast, the second half of low to high they really pick up (this is the good time to sail) and then the first half of high to low the winds stay fast but the waves really pick up and it becomes too dangerous. This is just an example though of course, if you have the luxury of winds consistant with tides where you are, you might need to observe the conditions relative to tide for a while and try to draw some conclusions.
If you have no tides, then i'm clueless as to how to know, just hope the weather channel is right I guess(hah! yeah right).